Russia’s military build-up is a likely precursor to the Group of 20 summit on Sept. 4 when Vladimir Putin, the Russian president, will inevitably meet Angela Merkel, the German chancellor, and François Hollande, the French president, to discuss Ukraine.

In Ukraine's restive east, artillery explosions ring out and international monitors record dozens of shell bursts every evening. The ceasefire between Kyiv and the Russian-backed separatists is fraying badly.

Signs of a flare-up in fighting began in July and Ukraine and its western allies are observing a more worrying development: a build-up of conventional Russian forces in Crimea and along the countries' shared border.

Russia has positioned military units with thousands of troops to the north in Bryansk, to the east near Rostov, to the south in Crimea and to the west in the separatist Moldovan region of Transnistria. There are signs they are preparing for fighting.

"Preparations for conventional conflict between Russia and Ukraine are accelerating, and the likelihood of open war is increasing rapidly," the Institute for the Study of War, a US think-tank, wrote in a report last Thursday.

On Monday that appeared to move a step closer. Sergei Lavrov, Russia's foreign minister, vowed that "exhaustive measures" would be taken against Kyiv in reprisal for what Russia called an attempted terror attack by Ukraine's defence ministry.

"These are very, very clear signals from Russia that say they are willing to escalate and to push boundaries," says Kathleen Weinberger, a Russia and Ukraine expert at the ISW. "We're seeing reports of big military convoys being sent into the separatist areas, Russian troops being sent to the border and a lot more naval equipment in the Black Sea - including some of their most advanced subs, which have quite powerful ground attack cruise missiles."

But, Ms. Weinberger adds: "A lot of this is still unknown. Russia is showing us a very overt and threatening 20 per cent but leaving 80 per cent of what they're doing unclear."

Russia is dialling the tension in the region up and down - as it has done in the past - ahead of another round of diplomatic negotiations, says one senior European intelligence official. The sabre-rattling is about reinterpreting the Minsk agreements in Russia's favour, the official says.

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